Laundry treating equipment with a driving motor mounted on the drum shaft

ABSTRACT

The invention relates to a laundry treatment apparatus like washing machines, laundry dryers or a washer-dryers with a rotatably mounted drum ( 6 ) with an at least approximately horizontal axle and with a drive motor ( 10 ) structured as a synchronous motor ( 10 ) energized by permanent magnets arranged on the drum ( 6 ) shaft, the stator ( 16 ) of the motor ( 10 ) being provided with a winding ( 18 ) which is energized by a converter. In order to optimize the motor in such machines in respect of energy consumption, noise development and costs it is proposed to design the winding ( 18 ) as a single pole winding, whereby the number of stator poles ( 27 ) and of the magnet poles ( 23 ) is different, and to utilize a frequency converter ( 104 ) as the converter the output voltage of which being set such the continuous currents are generated in all winding strands.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a laundry treatment apparatus, such as awashing machine, clothes dryer or washer-dryer with a drum with mountedfor rotations about an at least approximately horizontal axis and with adrive motor arranged on the shaft of the drum and structured as asynchronous motor energized by a permanent magnet the stator of which isprovided with a winding energized by a converter, the winding beingstructured as a single pole winding and the number of stator poles beingdifferent from those of the magnet poles.

2. The Prior Art

Washing machines of the kind referred to are generally known fromWO-A-98/00902. Washing machines are also known from DE 3,819,651 A1 inwhich the laundry drum is driven directly without the use of thecustomary intermediate transmission (drive belt, pulley). In such drivesthe rotor constitutes the component for transmitting rotational movementto the drum of the washing machine. Furthermore, DE 3,819,861 A1proposes to use an asynchronous motor with a squirrel-cage rotor. Such amotor is characterized by a relatively quiet movement, but it suffersfrom the drawback that because of the prevailing marginal conditions,such as, for instance, the large air gap and high pole construction inan asynchronous motor, good efficiency cannot be achieved. Yet inconnection with a frequently operated household appliance anecologically friendly, i.e. energy-saving operation, is desirable.

A motor for directly driving the drum has been described in DE 4.341,832A1. That motor is structured as a synchronous motor fed by a converter.No further statements are made as regards the type of motor.

Furthermore, washing machines are known which are provided with directlydriving motors structured as external rotor motors (DE 4,335,966 A1; EP413,915 A1; EP 629,735 A2). The rotor may be manufactured as adeep-drawn component, such as a plastic bell or as a compound structure.The structure of a deep-drawn component is advantageous since in it, theiron forms the magnetic yoke and a hub may be integrated for receivingthe bell. Among others, such a structure also constitutes an arrangementtypical of venting motors.

Direct current motors without collectors are used in the above-mentioneddirect drives for washing machines. See, for instance, WO-A-98/00902.The stator winding there described may be structured either as aconventional three-phase current winding with a winding pitch overseveral stator teeth or as a single pole winding with a winding around astator pole. In this type of motor, commutation is performed by powersemi-conductors. In such an arrangement, individual strands of thestator winding are energized by a d.c to a.c. converter in dependence ofthe stator position so that the excitation field rotates with the motor.In a treble stranded excitation winding current for the generation oftorque flows at any given time in two strands only, the third strandremaining unenergized. The temporal current flow in the individualstrands is block shaped or trapezoidal. For that reason, when switchingthe individual windings on and off, large current change velocitiesoccur which generate noises at the motor. Such noises are undesirable,however, in laundry treatment apparatus of the kind sometimes installedin living facilities (kitchen, bathroom).

In electronically commutated d.c. motors, Hall sensors, magnetictransducers or optical sensors are utilized for sensing the rotorposition. The mounting of such sensors and their appurtenant signallines involves additional costs. Moreover, sensors and lines are subjectto malfunctioning. A further drawback is that operating with fieldweakening is not easily accomplished in such self-controlled motorsenergized by permanent magnets. The large spread of torque andrevolutions between washing and spinning operations necessary in washingmachines usually results in large motor current spreads. For thatreason, it is necessary to install switchable or tapped windings, orelse the motor winding and the power semiconductors have to be sized forthe largest possible current.

Synchronous motors sinusoidally energized and controlled by a converterare already known as servo-motors. They are utilized where precisepositioning is required. In known servo-motors the stator winding is aconventional three-phase current winding, and the number of rotor andstator poles is identical. While the three-phase current winding ischaracterized by conventional and known winding techniques, the largeamount of copper in the winding heads is a disadvantage as it not onlyincreases manufacturing costs but also the structural depth of themotor. The latter aspect would, in washing machines with a housing ofpredetermined depth, reduce the volume of the drum. Moreover, for acontrolled operation servo-motors require very accurate and expensivesensors for sensing the rotor position.

A further disadvantage of all previously mentioned motors with permanentmagnet excitation is their lack of field weakening, since the magneticflux of the motor essentially depends upon the field of the permanentmagnets and is, therefore, constant. For washing machine drives suchmotors are, therefore, rather unsuited since a large spread of torqueand revolutions between washing operations and spinning operations wouldentail a large spread of the motor current. The motor winding and thepower semiconductors of the frequency converter would, therefore, haveto be dimensioned for the largest current and would be very expensive.As an alternative, the windings could be tapped which would, however,require installing additional lines from the motor to the electroniccomponents. Also, expensive switching relays would be required.

OBJECT OF THE INVENTION

Therefore, it is the the object of the invention is to optimize, in alaundry treatment machine of the kind mentioned hereinbefore to optimizethe motor in respect of energy consumption, low noise development andcosts.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a laundry treatmentapparatus having a drum rotationally mounted on a substantiallyhorizontal axle and a synchronous motor with permanent magnets and astator including winding strands energized by a frequency converter theoutput voltage of which is set such that continuous currents aregenerated in all strands.

In contrast to hitherto known direct drives for washing machines withd.c. motors without commutators, all three winding strands of thethree-phase excitation winding are continuously energized in the driveconcept here described, with the frequency of the excitation field beingdetermined by the electronic control. In this case, the motor isoperated as an externally controlled synchronous motor. In connectionwith a synchronous motor with permanent magnet excitation this methodensures that the noise developed is very low.

By utilizing a single pole winding, copper consumption is less than in aconventional three-phase current winding; the volume of copper of thewinding heads is markedly less. Accordingly, the entire drive becomessmaller and more compact. Because of the smaller amount of copper and asa result of the lower copper losses higher degrees of efficiency can beachieved at the same motor size.

It is advantageous to structure the rotor as an external rotor. In thismanner, the most compact structural shapes may be obtained because thetorque generating radius of the air gap is located near the outerradius.

Furthermore, it is of advantage to utilize a control device whichregulates the output voltage of the frequency converter by a controlsuch that a minimum sinusoidal current is derived as a function of theload torque. Sinusoidal currents affect a very quiet motor movement anda reduction in losses resulting from current ripples. This isparticularly true where the output voltage is set as a sinusoidal pulsewidth modulation. Moreover, the torque-dependent current control ensuresan optimum degree of efficiency at each load point.

In synchronous motors with single pole windings the number of magnetpoles characteristically deviates from the number of stator poles. Aratio of rotor poles to stator poles of 2 to 3 or of 4 to 3 is favorablein a treble stranded arrangement and continuous energization or in arotational magnetomotive force of the stator winding. In these two casesonly does the vectorial addition of the voltages induced in theindividual pole windings yield a maximum and optimum degree ofefficiency.

At a pole ratio of 4 to 3, the use of thirty stator poles is favorablein order to cover the required range of revolutions from 0 to 2,000 min.The selected number of poles ensures a definite start-up at an externalcontrol, low torque ripples and a large spread of revolutions.

Aside from this, it is advantageous to base the control device forcontrolling the motor current upon a mathematical model of the motor andto energize the winding strands without rotor position transducers.Since motor current and voltage at the motor may be detected at thefrequency converter, there is no need for sensors at the motor.

In an advantageous embodiment of a control without sensors themathematical model may be calibrated either as required or continuously.Motor-specific parameters such as winding resistance, motor inductanceand the constant of the induced voltage may be detected by means of thecurrent sensors and microprocessor control present in the frequencyconverter and the mathematical model may be adjusted on the basis of themeasured values.

The essential advantage of the laundry treatment apparatus structured inaccordance with the invention derives from the possibility ofdimensioning the number of windings of the stator windings such that thelevel of the induced voltage or of the synchronous generated voltage forhigh revolutions is higher than the maximum output voltage of thefrequency converter. Such a winding design makes possible a fieldweakening operation of the synchronous motor in the range of higherrevolutions. The advantage of such a winding design is a markedreduction of the motor current in the washing mode. It may be selectedin such a manner that the motor may be operated with the same current inthe washing and spinning modes. Owing to the lower motor current smallerand less expensive power semiconductors may be utilized. Moreover, thelosses in the power semiconductors are reduced so that the overalldegree of efficiency of motor and power electronics is higher than incomparable drives utilizing the same quantity of copper. In order alsoto utilize field weakening when using a control with rotor positiontransducers, it is advantageous not to evaluate them at higherrevolutions. At higher revolutions, large and short-term load deviationsdo not occur so that controlling the motor current is not absolutelynecessary. In that case, the motor is operated with external controlswith voltage and frequency being determined by the converter regardlessof the position of the rotor field. The motor current will in suchcircumstances adjust itself within limits as a function of the loadtorque. In order to prevent an overload and an asynchronization of themotor, it will suffice to monitor the level of motor current as afunction of the frequency of the rotational field.

Furthermore, It is also possible by field weakening to achieve goodefficiency with high pole synchronous motors with permanent magnetexcitation at high revolutions as the losses resulting from magnetichysteresis are reduced as a result of field weakening.

Operation of d.c. motors without collectors with field weakening ispossible only at great complexity as in such arrangements it would benecessary to change the position of the rotor position transducer ormathematically to shift the instants of commutation. For the abovereasons, field weakening operation of servomotors is not known.

DESCRIPTION OF THE SEVERAL DRAWINGS

The novel features which are considered characteristic of the inventionare set forth with particularity in the appended claims. The inventionitself, however, in respect of its structure, construction and lay-outas well as manufacturing techniques, together with other objects andadvantages thereof, will be best understood from the followingdescription of preferred embodiments when read in connection with theappended drawings, in which:

FIG. 1 is a schematic view in section of a washing machine built inaccordance with the invention;

FIG. 2 is a partial section of the rear portion of a washing watercontainer, a drum and their drive motor;

FIG. 3 is a perspective presentation of the support cross of a washingmachine;

FIG. 4 shows an individual laminate of a stator of the drive motor;

FIG. 5 is a perspective presentation of a permanent magnet rotor;

FIG. 6 depicts a block circuit diagram of the structure of thecontrolled drive with three-phase current synchronous motor and rotorposition transducers; and

FIG. 7 depicts a block circuit diagram of the structure of the drivecontrolled without sensors with three-phase current synchronous motor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The washing machine shown in FIG. 1 is provided with a housing 1 withinwhich a wash water container 2 is suspended by springs 3 for oscillatingmovements. To dampen the oscillations relative to the bottom of thehousing 1, it is supported by friction dampeners 5. Within the washwater container 2 a drum 6 for receiving laundry (not shown) isrotatably supported. Drum 6, wash water container 2 and the fronthousing wall 1 a are provided with aligned openings through which thelaundry may be put into the drum 6. The openings may be closed by a door7 arranged on the front housing wall 1 a. Latching the door 7 is carriedout by an electromagnetic latching device 8. The door latching has onlybeen shown schematically in the drawing. Construction and function of anelectromagnetic latching device 8 as such is known from theabove-mentioned DE-OS 1,610,247 or from DE 3,423,083 C2 and will,therefore, not be described in detail. In the upper portion of the frontwall 1 a of the housing there is provided an operations panel in which arotary switch serves to select washing programs. As is known, thewashing programs include a washing cycle and a rinsing cycle subsequentthereto. The washing revolutions in household washing machines arebetween 20 and 60 per minute, the spinning revolutions, particularly atthe final spinning toward the end of the rinsing operation should be ashigh as possible. It is upwardly limited by the extent to which theoscillating system consisting of the wash water container 2, suspension3, drive motor 10, drum 6 may be loaded, the limits being at present atabout 1,600 revolutions per minute.

FIG. 2 depicts a partial section through the rear portion of a washwater container 2, a drum 6 and their drive motor 10. A four-armedsupport cross 11 shown in FIG. 3 is affixed to a marginal abutment 2 aformed by the circumferential wall 2 b of the wash water container 2 anda crimped portion of its bottom 2 c. A bearing hub 12 having two radialroller bearings 13 a, b inserted therein is provided in the center ofthe support cross 11. The roller bearings 13 a, b, in turn, serve toreceive a drive shaft 14 which is affixed to the bottom 10 of the drum.The rear end of the drive shaft 14 protrudes from the bearing hub 12. Apermanent magnet rotor 15 structured as an external rotor is mountedthereon and, therefore, drives the drum 6 directly. The stator 16 of thedrive motor 10 is affixed to the support cross 11.

The laminated stator core 17 including the stator windings 18 is ofsubstantially annular configuration. For mounting the laminated statorcore 17 on the support cross, the individual laminates 17 a are providedwith fastening eyelets arranged at the internal peripheral surface andprovided with through-bores 19. Fastening screws (not shown) are seatedin these through-bores 19 and threaded into threaded bores 26 in thesupport cross 11. The bores 26 are arranged concentrically with respectto the bearing hub 12. Their free ends are provided with supportsurfaces 20 for the frontal surface of the laminated stator core 17. Thelaminated stator core 17 is centered by radially formed reinforcementribs 21.

The rotor 15 consists of a pot-shaped deep-drawn component or aninjection molded aluminum component 15 a provided with a hollowcylindrical section 15 b containing the iron magnetic yoke 22 and, asrotor poles, the permanent magnets 23 mounted thereon (see also FIG. 5).Furthermore, the rotor 15 is provided with a hub 24 which is keyed and,therefore rigidly connected, to the free end 14 a of the drive shaft 14by a threaded bolt 25 and splines (not shown).

The drive motor is structured as a three-phase current synchronous motorexcited by permanent magnets. A treble-stranded single pole winding(tooth winding) is housed in the stator 16, the strands being connectedin a star connection (see FIGS. 5, 6). The windings of a strand on eachtooth 27 are series connected. Hence, the drive motor is structures as amodular permanent magnet machine. The ratio of rotor poles a to statorpoles 27 is 4 to 3 at thirty stator poles 27.

FIG. 6 is a block circuit diagram of the structure of the controlleddrive with a three-phase current synchronous motor 10. The number ofrevolutions of the motor 10 is preset at a desired value by the programcontrol of the washing machine ST 101 as a function of a programselected by means of the dial switch 9 (see FIG. 1). In order toinfluence the number of motor revolutions it is necessary to adjust thefrequency of voltage and current as well as the level of the voltage inthe stator windings 18. To control the motor the motor current isadditionally set in dependence of the load torque. To this end, at leasttwo strand currents I₁ and I₂ are measured by current sensors 103 a, b.

The adjustment of the previously mentioned parameters is performed bythe frequency converter 104. For this purpose, network voltage isinitially converted to d.c. by a rectifier 105 and is smoothed by abuffer capacitor 106. The d.c. voltage is converted by a three-phaseinverter 107 the output of which is connected to the stator winding 18.Since the buffer voltage is constant, the voltage at the motor 10 willbe set by way of pulse width modulation. The effective value of thevoltage may then be set by way of the pulse width. A pulse pattern willbe chosen which will lead to sinusoidal currents within the statorwinding 18 of the motor 10. This is referred to as sinusoidal pulsewidth modulation. The sinusoidal currents provide for very quiet runningof the motor 10 as well as for reduced losses otherwise caused bycurrent harmonics. To affect the pulse pattern, a microprocessor control108 with an integrated control 109 and a valve control 110 is associatedwith the inverter 107.

Calculation of the control signals for the transistors of the inverter107 is performed on the basis of the position of the rotor at any giventime in order to set the optimum orientation and force of the rotaryfield and thus to ensure sufficient torque at the rotor 15. A continuousand precise recognition of the rotor position are required because ofthe sinusoidal current supply of the synchronous motor 10 and the torquedependent current control. Resolvers or analog Hall sensors 111 may beused for this purpose. Hall sensors 111 are preferred because of theirlower prices. In both cases, the measuring systems are absolute andfurnish exact data about the absolute position of the rotor 15 relativeto the stator 16 immediately upon being turned on. Where two Hallsensors 111 are used they will generate two signals which arephase-shifted by 90°, with the assistance of the rotor magnets. Therotor angle may be determined on the basis of these two signals by themathematical function β=arctan(a/b).

Where analog Hall sensors 111 are used their self-calibration isrecommended since because of deviations between different sensors inrespect, for instance, of sensitivity, offset, temperature drift and soforth the analog output signals of different Hall sensors 111 in amagnetic field are not necessarily identical. A precise recognition ofthe rotor position thus requires the output signals to be corrected. Thecorrection aims at identical output signals in a magnetic field from theused Hall sensors 111. Such a correction may be carried out by storingthe analog output signals of both Hall sensors 111 during a rotorrevolution in a correction device 112 integrated in the microprocessorcontrol and by thereafter deriving from the stored values the mean valueas well as maximum and minimum values. Once the mean value is known, anyoffset may be corrected, whereas sensitivity and temperature drift maybe corrected on the basis of the maximum and minimum values. It is notnecessary to consider the influence of temperature on the remanenceinduction of the magnets 23 since in that case the output signals ofboth Hall sensors 111 are changed in the same manner and to the sameextent. Where the rotor angle is calculated on the basis of themathematical formula β=arctan(a/b) the quotient (a/b) will remainconstant at temperature induced changes of the magnetic field.

FIG. 7 is a block circuit diagram of the structure of a control in whichsensors for the recognition of the rotor position may be dispensed with.When controlling the synchronous motor 10 with a continuous, especiallysinusoidal current supply the position of the rotor must be calculatedby the microprocessor 108. This is carried out on the basis of amathematical model 103 of the motor 10 stored in the control in whichthe characteristic parameters of the motor such as winding resistance,motor inductance and induced voltages must be known. The motor currentsI₁ and I₂ and the motor voltage U_(—w) are continually registeredvectorially, i.e. according to amount and phase position, whereby thecurrents are measured by the sensors and the voltage is known from thepulse pattern generated by energization of the valve control 110. Inthis manner, the operational point of the motor 10 at any given instantmay be precisely defined, and the motor 10 may be operated at theminimum current required for the load torque. Since motor current andvoltage at the motor 10 are detected in the frequency converter 104 nofurther sensors are necessary at the motor 10.

In an advantageous embodiment of the control without sensors theparameters of the mathematical model 113 are adjusted either as requiredor continuously. Such an adjustment may become necessary if themotor-specific parameters (winding resistance, motor inductance andinduced voltage) change as a result of the motor 10 heating up duringoperation. The winding resistance and the induced voltage in particularare parameters strongly dependent upon temperature. By briefly feedingd.c. current into the stator winding 18 from the frequency converter104, preferably during the reversing pauses in the washing mode, theinstantaneous winding resistance (and, hence, the temperature of themotor) as well as the motor inductance may be determined provided thevoltage at the motor is known and the current is measured by the sensors103 a, b in the frequency converter 104.

The winding resistance R may be derived from the relation R=U/I and theinductance L from the time constant T=L/R, it being necessarycontinuously to measure the current in order to determine the timeconstant T.

Since the machine is being operated as an externally controlledsynchronous motor 10 a low output frequency of the frequency converter104 at start-up of the motor 10 is important. Typical switch-onfrequencies are from 0.1 to 1 Hz. In connection with the high number ofpoles of the motor 10 this ensures a definite start-up without bucking,even under a load.

The number of windings of the stator winding 18 is calculated such thatat higher revolutions the synchronous generated voltage and the inducedvoltage of the synchronous motor 10 are higher than the output voltageor the buffer voltage of the frequency converter 104. Such anarrangement allows an operation with field weakening at higherrevolutions. The field weakening makes it possible to operate the motor10 at about the same motor current in two different working conditionsat different revolutions and different torques, for instance in thewashing and spinning modes.

In this context, field weakening is to be understood as a weakening ofthe field generated by the permanent magnets 23 of the rotor 15 in theair gap by a field of corresponding force and phase position generatedin the stator 16. At the occurrence of field weakening the synchronousgenerated voltage and the motor current are not in phase; rather, thecurrent in the strands is ahead of the synchronous generated voltage. Atfield weakening, the angle between the stator magnetomotive force androtor field exceeds 90° (electrically). In addition to its forcegenerating component in the transverse axis the current has a negativelongitudinal component in the stator which is opposing the rotor field.The current in the strands may be vectorially divided into a forcegenerating and into a field generating component with the forcegenerating component being in phase with the synchronous generatedvoltage and the field generating force opposing and weakening the rotorfield.

In a controlled operation the torque generating component of the currentin the transverse axis and the longitudinal current component in thestator may be adjusted separately from each other by means of thecurrent sensors 103 a, b which will detect the strand current in atleast two phases. Hence, the drive may be operated at minimum currentand optimum efficiency even in the field weakening range. Sensing andcontrolling the motor current in a field weakening operation arerecommended since at too large a negative longitudinal current componentin the stator the magnets may become irreversibly weakened by the fieldgenerated by the magnetomotive force.

In a sensorless control the rotor position or the position of the rotorfield is calculated on the basis of the measured strand currents and themathematical model 113 the motor 100. The rotor position may thus bedefined only as long as the motor is energized. For that reason, it isadvantageous in a sensorless control to maintain the motor 10 energizedeven during its phase of deceleration from the washing revolutions orfrom the spinning revolutions to complete stoppage. During this processthe rotary field defined by the frequency converter 104 is continuouslyreduced in frequency and amplitude until complete stoppage has beenreached. If the winding strands of the motor 10 are at least partiallyenergized even during stoppage, thereby to maintain the position of therotor 15, the next start-up into the defined direction may commenceimmediately and without bucking. If Hall sensors are utilized,deceleration may take place without control or without feeding ofcurrent.

The described drive makes possible reversals without any or no more thana short reversing pause. In washing machines equipped with a drive beltas an intermediate drive this would not be possible without somedifficulties. The drives usually utilized in such washing machines areuniversal motors which decelerate without controls and without braking.After switching off such a motor the washing drum will slow down orcease oscillating. To prevent increased wear and noises of the drivebelt it is necessary following switching off to wait until the drum hascome to a definite stop before the motor can be switched on again. Inwashing machines with drive belts these stopping intervals typicallylast 2 to 4 seconds. By eliminating these hitherto customary and neededpauses during reversing operations washing cycles of reduced durationwill result.

A further advantageous embodiment of a laundry treatment apparatus isprovided with a device for evaluating the voltage induced by thedeceleration of the rotor 15. The revolutions at any given instant maybe deduced from this voltage. As long as the motor 10 is rotating avoltage will be induced in the stator winding 18 of the motor 10. Leveland strength are in proportion to the number of rotations. The inducedvoltage may be utilized to sense drum rotation. In a washing machinewith an electromagnetically or electromechanically latched door theinduced voltage may be used to operate the latching device. It is thuspossible in a simple manner to provide for safe latching of the door 7without use of additional revolution sensors. Such an application ispossible in general in washing machines provided with rotors excited bypermanent magnets and is thus not limited to the embodiment inaccordance with the invention.

Having described our invention, what we claimed is:
 1. An apparatus fortreating laundry, comprising: a drum; a substantially horizontal axlefor rotatably mounting the drum; a synchronous electric motor comprisinga rotor comprising a predetermined number of permanent magnets and astator including a number of stator poles differing from the number ofmagnets and a single pole stator winding including a predeterminednumber of strands; and a frequency converter for energizing the statorwinding and providing an output voltage for generating continuouscurrents in all winding strands.
 2. The apparatus of claim 1, whereinthe rotor is structured as a rotor external of the stator.
 3. Theapparatus of claim 1, wherein rotation of the drum is subject to loadtorque and wherein the converter is provided with means for controllingthe output voltage to generate minimum sinusoidal motor current as afunction of the load torque.
 4. The apparatus of claim 3, wherein theoutput voltage is set as a sinusoidal pulse width modulation.
 5. Theapparatus of claim 3, wherein the means for controlling is based upon amathematical model of the motor and wherein the winding strands areenergized by currents in the absence of rotor position sensors.
 6. Theapparatus of claim 5, further including sensors for detecting values ofmotor-specific parameters of winding resistance, motor inductance and aninduced voltage constant and wherein at least one of the correspondingparameters of the mathematical model is adjusted in accordance with thecorresponding detected parameter.
 7. The apparatus of claim 6, whereinin a predetermined mode of operation the rotor may be positioned by acontrolled deceleration and reversed substantially immediately followingstoppage.
 8. The apparatus of claim 1, wherein the stator winding isstructured as a treble stranded winding and wherein the ratio ofpermanent magnets to stator poles is 2/3.
 9. The apparatus of claim 1,wherein the stator winding is structured as a treble stranded windingand wherein the ratio of permanent magnets to stator poles is 4/3. 10.The apparatus of claim 1, wherein the number of stator poles is aboutthirty.
 11. The apparatus of claim 1, further comprising two Hallsensors for energizing the winding strands by analog output signals, anda correction device for calibrating the analog output signals in respectof their time and conditions depending deviations.
 12. The apparatus ofclaim 1, wherein the number of stator windings is dimensioned such thatthe level of one of an induced current and a generated synchronousvoltage is greater than the output voltage of the converter.
 13. Theapparatus of claim 1, wherein means is provided for field weakening toenergize the motor at higher revolutions in the absence of rotorposition sensors.
 14. The apparatus of claim 1, further comprising meansfor evaluating the voltage induced by the rotor.
 15. The apparatus ofclaim 14, wherein the drum is provided with an electrically latchabledoor and wherein the door is latched by the evaluation means.